This invention relates to a method of dynamic matrix control for a process or series of processes in an industrial environment, for example, in a petroleum refining or processing plant.
The dynamic matrix control method described in U.S. Pat. No. 4,349,869 is a multivariable control technique incorporating feedforward and feedback control. The feedforward aspect is included by prediction of the future response of the process to changes in both manipulated variables and measurable disturbances, while feedback is accounted for by modification of this prediction through readback from the process. The prediction of process responses to changes in inputs is based upon previous plant testing, during which changes in the manipulated variables are introduced and the resulting changes in the controlled variables are recorded. Constraints on the size of moves and high or low limit checking on the manipulated variables is also included in the technique. If a limit is met in one manipulated variable, this variable is no longer used for control and the remaining manipulated variables are modified to take this into account and control the process.
The prior art method of dynamic matrix control determines a set of moves for the manipulated variables which will minimize the deviation of the controlled variables from their respective set points over a predetermined interval. However, it does not provide for constraints or limits on controlled variables. Therefore, the set point for the controlled variables cannot be placed at or near a limit point for that controlled variable, since the limit may be exceeded during the deviation of the controlled variable about its set point as the manipulated variables are moved. Generally, it has been the practice to move the set point from the minimum or maximum limitation by a safety margin to ensure the safety of personnel and equipment and continuity of the process. This safety margin usually results in lower efficiency or yield from the process. In addition, the prior art method of dynamic matrix control could not directly determine a set of moves for the manipulated variables to drive the controlled variables to their respective set points and limit the manipulated variables to a predetermined limit. Rather, the prior art method determined a set of moves and then checked to see if the manipulated variables would violate a limit. If one of the manipulated variables would violate a limit if the moves were implemented, it was necessary to determine a new set of moves for the manipulated variables. The new set of moves would have to be limit checked before it could be implemented. This procedure would have to be repeated until a set of moves is determined which does not violate the limits for the manipulated variables.
Therefore, it is an object of the present invention to provide a method of dynamic matrix control which limits one or more of the controlled variables to a maximum/minimum limit.
In addition, it is an object of the present invention to provide a method of dynamic matrix control which determines a set of moves for the manipulated variables which drive the controlled variables towards their set points and prevents one or more of the manipulated variables from exceeding a predetermined limit.